Non Isothermal Tube Model Characterization for High Speed Thermoforming Stretch
Accurate non isothermal tube characterization reduces thermoforming wall variance to under six percent at hundred hertz stretch rates.
Planar Strain Hardening
Meaning
Resistance to additional deformation grows as a polymer film or sheet undergoes biaxial stretching during production. The phenomenon of planar strain hardening occurs when molecular chains align and lock in the direction of the stress, increasing the force required for further drawing. This resistance prevents localized thinning in areas that experience higher tension, ensuring a more uniform distribution of material thickness across the final component.
Material Geometry
Molecular orientation dictates the mechanical response of semi-crystalline resins during the thermoforming process. Planar strain hardening stabilizes the wall thickness of a part by inhibiting excessive thinning in deep draw zones. Manufacturers observe this effect when the resin reaches its natural draw ratio.
Resins that lack sufficient chain entanglement fail to generate enough internal resistance, resulting in immediate failure at high stress points.
Tooling Dynamics
Tool temperature control regulates how quickly the material builds sufficient structural integrity to resist further deformation. High melt strengths associated with planar strain hardening allow the molder to maintain tighter tolerances during high-speed vacuum forming. Lower processing temperatures increase the rate at which the resin develops this resistance, although excessive cooling traps internal stresses that cause warpage.
Uniform heating of the initial blank remains necessary to ensure the onset of hardening happens symmetrically across the entire geometry.
Production Variance
Variations in regrind percentages degrade the long-chain branches that support the hardening effect under tension. Virgin pellets provide a predictable response because molecular weight distribution remains consistent across each batch. Parts produced with high levels of reprocessed material exhibit inconsistent wall thicknesses because the hardening threshold shifts compared to the base specification.
Consistent mechanical performance relies on the purity of the resin input and strict adherence to defined heat profiles.